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It depends on what kind of attack it is. Check out the Swarm Defenses rule.
In short: yes, if it's an attack that only targets one creature, yes, that swarm is immune.
If it's an attack that targets multiple but finite creatures, like from a weapon with the Line or Automatic property, then it takes damage normally.
If it's an attack that targets everything in an area, like an explode weapon or area of effect damaging spell, then it takes +50% damage.
(I'm not sure why Line is in the "regular damage" category, not the "x1.5 damage" category, but there too have it.)
Line weapon property being mentioned is probably because of line weapons don't work like line effects and instead can stop midway, but that doesn't feel like a "good" reason.
Normally no. It's for when you're dealing with things like swarms of locusts, swarms of microraptors, swarms of robot ants, etc.